Fuel cells such as PEFC and DMFC are still under development, and there has been no established method for testing, evaluating, and diagnosing these fuel cells. At present, therefore, an alternate current impedance or output voltage-output current characteristics of a fuel cell are measured by activating the battery by a fuel/oxidizer gas supply control device. The fuel cell is shipped after it is confirmed that the measured alternate current impedance or output voltage-output current characteristics satisfy development and product specifications.
Especially, there has been no method for performing a life test and deterioration prediction of the battery other than actually operating the battery for its lifetime according to the product specification, because factors that accelerate deterioration of battery is still unknown.
Consequently, it requires a considerable amount of time to confirm life or deterioration of fuel cell operation. For example, in order to test fuel cell life by means of sampling, it is necessary to drive the battery for its lifetime according to the product specification.
In order to find the life of fuel cell operation, it is necessary to measure its impedance and output voltage-output current characteristics at predetermined intervals. It requires considerable ingenuity to make use of the measured impedance and output voltage-output current characteristics in judging life or other properties of the fuel cell, because changes in these static characteristics are expected to be very small. Accordingly, it will become easier to judge the life of the fuel cell if its dynamic characteristics can be used for the judgment.
The following describes a method for testing, evaluating, and diagnosing a secondary battery, referring, as examples, to FIG. 13(a), which illustrates manufacturing steps for a lithium-ion battery, and FIG. 13(b), which illustrates steps of charging and discharging the battery in the manufacturing steps.
The manufacturing process for a lithium-ion battery are roughly divided into (1) an electrode mixing step, (2) a binding step, (3) an injection step, (4) a post-assembling step, (5) a charging and discharging step, and (6) a pre-shipment test step.
In the charging and discharging step (5), as shown in FIG. 13(b), includes the following procedures: [1] impedance measurement; [2] voltage measurement; [3] charging; [4] discharging; [5] capacitance ranking; [6] charging; [7] a self-discharge test; and [8] pre-shipment charging and discharging.
In the charging and discharging step, the steps [3], [4], [6], and [7] take a long time. In step [3], it takes three hours to fully charge the battery. In the step [4], it takes an hour to completely discharge the battery. At present, it is impossible to shorten the time required for these steps, because a full charge state of the battery cannot be simulated, for example, by half charging it.
Although not shown in FIG. 13(b), the properties of the battery are sometimes measured after charging and discharging the battery once, and then repeating charging and discharging a hundred times, so as to confirm reliability of the battery before shipment. In this case, the charging and discharging step takes at least 400 hours.
In view of the above problems, attempts have been made to shorten the time required for testing and diagnosing a battery by diagnosing a state of the battery according to a system identification theory. Examples of such attempts are disclosed in Japanese Publication for Unexamined Patent Application No. 232273/1998 (Tokukaihei 10-232273; publication date: Sep. 2, 1998), Japanese Publication for Unexamined Patent Application No. 337282/1994 (Tokukaihei 6-337282; publication date: Dec. 6, 1994), and the like.
For example, Tokukaihei 10-232273 discloses a method for determining remaining amount of electricity in a battery by applying an alternate current signal to the battery to be analyzed, estimating a transfer function of the battery from a result of sampling alternating voltage and alternating current, and calculating an extreme value of the transfer function.
However, according to the method of Tokukaihei 10-232273, in a device for measuring alternating voltage and alternating current of the battery, a voltage source noise (voltage load) with a serial load of an impedance element are serially connected to the battery. This is not an optimal method for extracting battery characteristics because an error of the impedance element directly affects the measurement.
Specifically, in Tokukaihei 10-232273, in obtaining the transfer function of the battery, an impedance is identified by including a series impedance (i.e. transfer function G(s), which is determined from the measured alternating voltage and alternate current, is expressed as a sum of transfer function HB(s) of the battery and transfer function HI(s) of the impedance element.)
Accordingly, in order to obtain an actual impedance of the battery, it is necessary to subtract the external impedance from the measured impedance. In Tokukaihei 10-232273, resistance and pure capacitance are used as the impedance element. The resistor causes a problem when there is a temperature change, and the capacitance, by nature, has a large margin of error. Therefore, the errors directly affect accuracy of identification. If measurement is to be performed by directly connecting the voltage source to the battery without using the impedance element, a current value becomes excessively large, and it becomes difficult to control the current.
There is also a subordinate problem that it is necessary to switch between the resistor and the capacitor.